1. Field of the Invention
The present invention relates to a process for producing diamines which are useful as, for example, starting materials for various polyamides and polyurethanes.
2. Description of the Background Art
Several processes for producing diamines are known, including feeding a solution obtained by dissolving a dialdehyde in a solvent to a reactor where a hydrogenation catalyst, the solvent, ammonia and hydrogen are present, and subjecting the dialdehyde to reductive amination to obtain the corresponding diamine. These processes include: (1) introducing a dialdehyde into a reactor for reductive amination at a rate which is not substantially higher than the consumption rate of the dialdehyde (see U.S. Pat. No. 2,636,051); (2) hemiacetalizing a dialdehyde by dissolving it in an alcoholic solvent and introducing the resulting solution into a reactor for reductive amination (see Japanese Patent Application Laid-open No. 17413/1993); and (3) mixing a dialdehyde with a diluting agent such as an alcohol while maintaining the mixing temperature at not more than 5.degree. C., thereby suppressing formation of hemiacetal, and then feeding the mixture obtained to a reactor for amination (see Japanese Patent Application Laid-open No. 69999/1995).
In addition, one of the present inventors has found (4) a process which comprises, in the reductive amination of a dialdehyde effected by feeding a solution obtained by dissolving the dialdehyde in a solvent to a reactor where a hydrogenation catalyst, the solvent, ammonia and hydrogen are present, using a nickel catalyst carried on an inorganic oxide as the hydrogenation catalyst. In this process the catalyst may be re-used and can produce diamines in one step and at a high yield and low cost. See Japanese Patent Application Laid-open No. 196586/1995.
The above process (1) discloses Examples where water and Raney nickel are used as the solvent and catalyst, respectively. This process, however, where the yield of diamine is as low as not more than 40%, cannot be a commercial process in view of economy.
The above process (2) discloses an example where methanol or ethanol and Raney nickel are used as the solvent and catalyst, respectively, and can produce a diamine at a high yield of about 90%. However, according to a follow-up test conducted by the present inventors (Comparative Example 7 of the present specification), the Raney nickel used in the reaction markedly decreases in its catalytic activity, thereby causing problems when the recovered catalyst is to be re-used. This process cannot be economical in view of catalyst cost.
The above process (3) requires temperature control to suppress formation of hemiacetal. Cooling with a coolant should become necessary to this end, which is not favorable commercially.
With respect to the above process (4), the specification discloses in its Example 10 the reaction results when the catalyst was re-used. However, there the catalyst was re-used only once. The specification does not describe about the reaction results when the catalyst was repeatedly re-used or used continuously. The present inventors have studied repeated re-use of the catalyst in the process (4) (Comparative Example 1 of the present specification). In this case, the diamine yield decreased from 92% on the first use and 91% on the second use to as low as 82% on the fifth use. Accordingly, with this process (4) the diamine yield decreases on repeated re-use of the catalyst, which means that the process has a problem when used in a commercial production of diamines where the catalyst is continuously used.
Accordingly, an object of the present invention is to provide a process for producing diamines from the corresponding dialdehydes in one step and at a high yield and a cost lower than that with the conventional processes.
Reductive amination generally produces water as a by-product, which means that water is present in the reaction zone. Among the above-cited literature, only (3) Japanese Patent Application Laid-open No. 69999/1995 refers to this water by-product. Even this literature gives only the description in its Example 1 "the reaction water that formed is distilled" on work-up procedure after the reaction. With this type of description, it cannot be considered that water is recognized as having any influence on the reaction results. Although the disclosure gives no description about the water concentration during the reaction, the concentration is estimated to be 3.3% by weight at maximum from reaction conditions described therein.
U.S. Pat. No. 2,636,051 (i.e., (1)), all Examples of which use water as solvent, naturally gives no description about the water by-product. With (2) Japanese Patent Application Laid-Open No. 17413/1993, the amount of water by-product is, from the reaction conditions described in its Example 1, found by calculation to be 3.4% by weight. With (4) Japanese Patent Application Laid-open No. 196586/1995, the amount of water by-product is, from the reaction conditions described in its Example 1, found by calculation to be 2.30% by weight.
Accordingly, it is clear that none of the processes described above pay attention to the influence of water by-product on reductive amination, or refer to control of the concentration of water present in the reaction zone.